Aminoacyl tRNA synthetases will be inactivated with oATP, phenylglyoxal and other reagents labeled with radioactive atoms (3H, 14C or 32P). The inactivated enzymes will be degraded by specific chemical and enzymatic methods and the labeled fragments will be purified and identified. Additional reagents will be used to modify specific amino acid residues and the effects of the modifications on the partial reactions (pyrophorphorolysis and transfer) will be determined. Attempts to crystallize synthetases will continue. Binding of reagents such as ATP to synthetases will be studied by NMR to determine the sites of interaction and the equilibrium constants for the interactions.